1. Field of the Invention
The present invention relates to an in-vehicle antenna apparatus mounted on a windowpane of a vehicle, such as an automobile, and particularly, to an in-vehicle antenna apparatus in which a conductor layer provided on one surface of a circuit substrate of an electronic circuit unit is opposed to a radiation conductor in order to achieve a higher gain.
2. Description of the Related Art
A conventional in-vehicle antenna apparatus is provided with a radiation conductor disposed on an inner surface of rear glass or front glass of a vehicle, and an electronic circuit unit that includes a pre-amplifying circuit and that is attached to the inner surface, such that the in-vehicle antenna apparatus is capable of, for example, receiving a circularly-polarized wave or a linearly-polarized wave sent from a satellite or a ground-based station. In comparison with other types of antenna apparatuses that are set on the exterior of a vehicle, such as a roof, this type of antenna apparatus is advantageous in having a longer lifespan and a lower possibility of being stolen. Furthermore, in comparison with antenna apparatuses that are set in the vicinity of the inner surface of a windowpane of a vehicle, this type of antenna apparatus is advantageous in providing a good space factor and a wide angle of view.
In an in-vehicle antenna apparatus of this type, the electronic circuit unit attached to the inner surface of a sheet of glass, such as rear glass or front glass, facing the inside of the vehicle includes a housing that houses a circuit substrate provided with, for example, a pre-amplifying circuit. In the electronic circuit unit, a radiation conductor having a predetermined shape and disposed on the sheet of glass is electrically connected with the circuit substrate via appropriate means so that the radiation conductor can receive electricity and load a received signal.
Japanese Unexamined Patent Application Publication No. 6-53722 (p. 2 to p. 3, FIG. 1) discloses an example of such a conventional in-vehicle antenna apparatus in which a conductor segment protrudes from an insulating housing that houses a circuit substrate such that the conductor segment is soldered on an electric feeding point of a radiation conductor. Since one end of the conductor segment is connected to an input section of a pre-amplifying circuit inside the housing, the radiation conductor and the pre-amplifying circuit are electrically connected to each other via the conductor segment, and the electronic circuit unit is fixed on the sheet of glass.
In such a conventional in-vehicle antenna apparatus, however, a radiation pattern is generated not only on one side of the sheet of glass having the radiation conductor but also on the other side of the sheet of glass. This may easily lead to an insufficient gain in the desired direction. In order to solve such a problem, a conductor layer functioning as a radio-wave reflective surface may be disposed in the interior of the vehicle at a position facing the radiation conductor. In that case, however, a high gain cannot be achieved unless the distance between the radiation conductor and the conductor layer is set in a highly accurate manner.
Furthermore, in this example of a conventional in-vehicle antenna apparatus, the electronic circuit unit, which is an integrated unit, is fixed to the sheet of glass by, for example, soldering. This is problematic in view of the fact that when the circuit substrate is to be inspected or replaced with a new one, it is necessary to perform complicated processes, such as demounting the electronic circuit unit from the sheet of glass and remounting the electronic circuit unit back to the sheet of glass, and thus makes the maintenance difficult.